STUDY OF STRUCTURE OF FULGURITES 68 1 



of about 3 /u. diameter, made up of concentric shells, which 

 exhibited a faint cross, remaining fixed on rotation. 



This unique occurrence of devitrification in the glass of a sand- 

 fulgurite is perhaps mainly connected with the high proportion 

 of feldspar in the original sand. Such a chemical constitution 

 must have approached that of the rhyolite ' used in the experi- 

 ments of Barus and Iddings, whose viscosity and whose electric 

 conduction, when fused, were both found to exceed those of the 

 less acid rocks, and led to the conclusion that "electric conduc- 

 tion increases with the degree of the acidity of the magma 



Since fusibility decreases in a marked way as the composition 

 of the magma approaches pure silica, it follows that, in a series 

 of different magmas, electric conduction at an}' given tempera- 

 ture increases in proportion as the viscosity increases." From this 

 it must be inferred that the more quartzose sands, when once 

 melted by a lightning stroke, have offered the viscous medium 

 of readiest passage to the current. 



Exception has been rightly taken, however, to the assump- 

 tion that the material of a fulgurite-tube was necessarily or 

 entirely " quartz-glass." ^ The general distribution of feldspar 

 in notable quantity through ordinary sands is a fact in favor of 

 the suggestion of Wichmann and Harting, that the minerals of 

 comparatively ready fusibility have served as flux and thus 

 ended in reducing the more refractor3^ Particularly then in 

 this fulgurite do we need to consider not only the relative fusi- 

 bilities of feldspar and quartz, but also the relative degrees of 

 their solvency in contact with the first formed portion of molten 

 glass, doubtless so instantaneous in formation as to be independ- 

 ent of all differences in fusibility, solution, conduction or any 

 other property of the various sand-grains. 



To throw light on this question and on the variations, 

 recorded beyond, in devitrification of such natural glasses, a 



' Silica, 75.5 per cent.; alumna, 13.25 ; soda, 4.76 ; potassa, 2. 85, etc. Heated in 

 a platinum crucible, it melted at 1500° C, was very viscous (" a stiff paste ") even at 

 1600°, and quite viscid at 1700°. Am. Jour. Sci., Vol. XLIV (1892), pp. 242-249. 



^See V. GiJMBEL, loc. cit., and idetn. Vol. XKXVI (1884), pp. 179-180, and criti- 

 cism by A. Wichmann. 



